Our analysis of the mechanisms by which the cyclic AMP content of brain cells is regulated will be continued along four lines: 1. The basic mechanism underlying agonist-induced desensitization will be sought. Attempts also will be made to isolate and characterize a putative intracellular intermediate that promotes desensitization. Attempts also will be made to detect agonist-specific induction of the phosphorylation of unique components that occurs concomitant with desensitization and that is lost concomitant with return of responsiveness. 2. The binding of 125I-HYP to Beta-receptors of cells from synchronously growing populations will be determined. Such information should indicate whether or not the change in responsiveness to ISO observed during growth is related to changes in binding properties of the Beta-receptor. 3. The mechanism whereby cholera toxin apparently increases the efficiency of coupling of hormone-receptor complex and adenylate cyclase will be studied. The effect of the "active" protomer of cholera toxin (which can activate adenylate cyclase in membranes) on hormonal responsiveness of membrane bound adenylate cyclase will be examined. 4. We will begin this year to isolate from human astrocytoma cells the three known components of the adenylate cyclase system; namely, the Beta-receptor, the GTP-binding protein and the catalytic moiety. BIBLIOGRAPHIC REFERENCES: Leichtling, B.H., Drotar, A.M., Ortmann, R. and Perkins, J.P.: Growth of Astrocytoma Cells in the Presence of Prostaglandin E1: Effect on the Regulation of Cyclic AMP Metabolism. J. Cyclic Nuc. Res. 2: 89-98 (1976). Su, Y.-F., Cubeddu, X.L. and Perkins, J.P.: Regulation of Adenosine 3':5'-Monophosphate Content of Human Astrocytoma Cells: Desensitization to Catecholamines and Prostaglandins. J. Cyclic Nuc. Res. 2: 257-270 (1976).